This invention relates to an infrared absorbing eyeglass lens substrate made of a polyurethane resin, and a method of manufacturing such a lens substrate.
Ordinarily, an eyeglass lens is formed by casting polymerization with a convex lens surface formed on the front side and a concave or flat surface formed on the back side. In order to adjust its lens power, only its back side is ground with its front side not ground. Then one or more coatings are optionally provided on the lens to improve its performance.
A polarized eyeglass lens substrate includes a polarizing element including a polarizing film and integrally superposed on the above-described eyeglass lens substrate. The polarizing film is formed by uniaxially stretching a resin film with a polarizing dye such as iodine impregnated.
In order to form a polarized eyeglass lens substrate by casting/polymerization, a polarizing film is pre-shaped so as to have a predetermined curvature and fitted in a ring-shaped gasket having a diameter corresponding to the diameter of the lens substrate so as to extend along the inner periphery of the gasket. Then, a pair of molds are provided over and under the polarizing film, respectively, liquid-tightly fitted into the gasket, and a monomer is poured into the cavity defined between the molds, thereby integrating the monomer with the polarizing element by polymerizing the monomer (see JP Patent Publication 2001-311804A).
Eyeglasses are useful not only to correct eyesight but also to protect eye cells from harmful light beams. For the latter purpose, many eye protecting means such as sunglasses and light shield eyeglasses use ultraviolet absorbing eyeglass lenses containing ultraviolet absorbing agents that prevent transmission of ultraviolet rays.
It is also required for eyeglass lenses to eliminate not only ultraviolet rays but also infrared rays.
Among infrared rays, electromagnetic waves in the wavelength range of 780 to 1300 nm, which are invisible, are called near-infrared rays, and electromagnetic waves in the wavelength range of 1300 to 2000 nm, which are also invisible, are called middle-infrared rays. Infrared rays in these wavelength ranges can penetrate as deep as 30 mm into human skin. Near-infrared rays in particular can penetrate the cornea and almost reach the retina, and thus could damage the eyeground.
Not only may human eyes suffer from thermal burns when exposed to intense light for a short moment, but damage may accumulate over a long period of time, causing e.g. retinopathy, and clouding of the crystalline lens (cataract).
Ordinary known infrared absorbing pigments for preventing transmission of infrared rays, i.e. pigments capable of absorbing infrared rays (also called “infrared absorbing agents”) include compounds of the azo family, aluminum family, anthraquinone family, cyanine family, polymethine family, diphenylmethane family, triphenylmethane family, quinine family, diimonium family, dithiol metal complex family, squarylium family, phthalocyanine family, and naphthalocyanine family.
An infrared absorbing filter is known which includes an infrared absorbing layer formed by applying and drying a solvent-containing coating liquid comprising a resin composition in which one or more of the above-mentioned infrared absorbing agents are dispersed in a binder resin (see JP Patent Publication 2005-43921A).
JP Patent Publication 2003-107412A discloses (in claims 2 to 4 and paragraphs [0013], [0020], [0022] and [0023]) an eyeglass lens made of a synthetic resin having excellent optical properties as eyeglasses, such as polycarbonate, diethylene glycol bisallyl carbonate (CR-39), or polymethylmethacrylate (PMMA) to which 0.001 to 0.05% by weight of one or more of the above-mentioned infrared absorbing agents.
But in these conventional arrangements, in order not to deteriorate the optical properties inherent to the eyeglass lens, the thickness of the coating layer formed on the surface of the lens and containing an infrared absorbing agent has to be as thin as possible. Such a thin coating layer cannot sufficiently absorb infrared rays.
In order to give a general-purpose lens the ability to absorb infrared rays, it may be also possible to disperse an infrared absorbing agent in a thermoplastic resin material, and melt the thermoplastic resin by heating to form the lens. But in this case, optical strains may develop in the lens. Also, since it is impossible to sufficiently filter out foreign matter in the resin, no high-quality lenses can be formed with this method.
As the thermoplastic resin material for eyeglass lenses, methyl methacrylate (MMA) resin and polycarbonate (PC) resin are preferentially used because they are highly transparent. But MMA is not sufficiently impact-resistant. PC is sufficiently high in impact resistance if it has a predetermined molecular weight. But PC requires a molding temperature of 250° C. or higher. At such high temperature, the infrared absorbing agent deteriorates and decomposes. Thus it is impossible to obtain a lens having both the ability to absorb infrared rays and impact resistance.
CR-39, which is a typical casting hardening type resin used for plastic eyeglass lenses, and medium refractive index resins (such as Corporex, made by NOF Corporation; refractive index: 1.56) contain allyl diglycol carbonate and are anaerobic thermosetting resins which harden using diisopropyl peroxydicarbonate (hereinafter referred to as “IPP”). Since the catalyst used, i.e. IPP is a peroxide, the infrared absorbing agent deteriorates and decomposes by reacting with the peroxide, and cannot sufficiently absorb infrared rays.
Thiourethane resins are known as high refractive index resins obtained by chemically binding isocyanate and polythiol (such as thiourethane resin MR-7, made by Mitsui Chemicals, Inc.; refractive index: 1.67). When an infrared absorbing agent was added to one of these resins, it deteriorated and decomposed by reacting with sulfur components or catalysts, so that it was unable to sufficiently eliminate infrared rays.
From the structural viewpoint too, conventional infrared absorbing polarized eyeglass lens substrates have various problems.
For example, as shown in FIG. 7, if an eyeglass lens is formed in which an expensive property improving agent such as an infrared absorbing agent is present throughout the entirety of the resin composition forming the lens substrate B, in order to form a lens having a predetermined lens power, most part, i.e. the portion b′ of the lens substrate B is removed by grinding with only the small surface portion b, which is shown by one-dot chain line in FIG. 7, left. Thus, when the portion b′ is removed, most of the expensive property improving agent is also removed and discarded.
Also, since a prescription lens has a varying thickness in the radial direction according to its lens power, the amount per unit area of the property improving agent such as an infrared absorbing agent, which is uniformly contained in the lens substrate before being ground, varies with the thickness of the lens. Thus, it is impossible to uniformly absorb infrared rays over the entire area of the lens. Also, since an infrared absorbing agent usually has a color tone, a single lens has different tints according to local differences in thickness. This makes it impossible to stably produce high-quality lenses of uniform specifications.
If resin layers containing property improving agents such as an infrared absorbing agent are simultaneously formed on both sides of the polarizing film by casting, one of the two resin layers is a thick one forming the eyeglass lens substrate configured to be ground for adjusting the lens power, while the other resin layer is a thin one. Thus, when forming such resin layers by casting, the resin material flows at different speeds on the two opposite sides of the polarizing film. This may cause the polarizing film to be twisted, which could in turn cause separation of the film from the gasket, or reversal of the film. Also the resin layers may develop portions where its density is not uniform, or striae. Thus makes the lens inhomogeneous, thus causing non-uniform refractivity and flickering of the light that passes the lens.
A first object of the present invention is to provide an infrared absorbing polarized eyeglass lens substrate of which the infrared absorbing agent contained in the lens substrate is scarcely removed when the lens substrate is ground to adjust its lens power, and is less likely to deteriorate or decompose under heat during molding, thus sufficiently maintaining the ability to absorb infrared rays, and of which the color tone is uniform.
A second object of the present invention is to provide a method of efficiently producing the above-mentioned infrared absorbing eyeglass lens substrate which prevents separation of the polarizing film from the gasket, reversal and undue movement of the film, and which is free of striae, i.e. non-uniformity in density of the resin.